The present invention relates to an ultrasonic probe, and more particularly to an array-type ultrasonic probe improved into high-sensitivity for use in medical diagnostics.
An ultrasonic probe connected to an ultrasonograph (an ultrasonic diagnostic equipment) is used in contact with the surface of the patient's body. In other words, with the probe in contact with the surface of the patient's body, the ultrasonograph sends electric signals to the ultrasonic probe, a piezoelectric element inside the ultrasonic probe vibrates and emits ultrasonic waves into the patient's body. The ultrasonic waves sent are reflected by a tissue in the body, received by the ultrasonic probe and converted by the piezoelectric element into electric signals. These electric signals are further converted into image signals, and a display shows an image of the body tissue.
For this piezoelectric element for sending electric signals, a piezoelectric element is in most instances one-dimensionally placed in a single-type probe or two-dimensionally placed in an array-type probe. The array type is capable of obtaining images with good fineness, so is widely used as an ultrasonograph for medical diagnostic examination.
On the other hand, for imaging, harmonic imaging technology using higher harmonics signals plays a greater and greater role as standard diagnostic imaging technology, because of its capability of obtaining images with good clarity that have not been available through conventional B-mode diagnosis.
Higher harmonics signals used in harmonic imaging technology have a lot of advantages over fundamental waves:
(1) Lower side-lobe level gives good S/N ratio and contrast resolution.
(2) Higher frequency leads to a thinner beam and better lateral resolution.
(3) Small degree and small fluctuation of short-distance acoustic pressure causes no multiple echo.
(4) The attenuation beyond focal point equals that of fundamental waves, so makes greater depth velocity possible than ultrasonic waves having higher harmonics frequency as fundamental waves.
For a piezoelectric element making up an array-type ultrasonic probe for harmonic imaging technology, a broadband piezoelectric element has been used. The method has been generally used in which the broadband property has a low-frequency domain for sending fundamental waves and a high-frequency domain for receiving higher harmonics waves. Besides, for a technique for enhancing sensitivity, the conventional wisdom tells “a technology in which minute columnar piezoelectric elements are fixed with an organic compound like epoxy resin to manufacture a probe, the probe is used as an element for emitting and receiving ultrasonic waves, and each columnar piezoelectric element is caused to longitudinally vibrate to obtain better sensitivity” (see Japanese Application Patent Laid-Open Publication No. Sho 63-252140).
In this technology, however, narrow-band ultrasonic waves are used so that the spectrum of ultrasonic waves for emitting fundamental waves can have little possibility of overlapping with that of ultrasonic waves for receiving higher harmonics. This gives rise to the problem that narrow-band ultrasonic waves, since in general being long-tailing ultrasonic pulse signals, undesirably affect the resolution in the depth direction.
Another concrete composition of the array-type ultrasonic probe for harmonic imaging technology has been presented by a probe having emission separate from receiving in which a piezoelectric vibrator for emission and a piezoelectric vibrator for receiving are implemented and placed separately (see Japanese Application Patent Laid-Open Publication No. Hei 8-187245 or Japanese Application Patent Laid-Open Publication No. Hei 11-276478).
In addition, the conventional wisdom tells “a technology in which fundamental waves are emitted and ultrasonic waves including higher harmonics are received by arraying a plurality of first piezoelectric elements having primary acoustic impedance to emit and receive ultrasonic waves whose center frequency is f1, thus by implementing a first piezoelectric layer, arraying a plurality of second piezoelectric elements having secondary acoustic impedance to emit and receive ultrasonic waves whose center frequency is f2, thus by implementing a second piezoelectric layer and by overlapping these first and second piezoelectric layers” (See Japanese Application Patent Laid-Open Publication No. Hei 11-276478). But with this technology it is still impossible to obtain sufficient sensitivity.
Further, an element for emitting and receiving ultrasonic waves is improved in sensitivity by the laminating of inorganic piezoelectric elements, the enhancing of the electric consistency with the driving circuit by decreasing the apparent impedance and the increasing of emission sensitivity by enlarging the electric field intensity applied to elements and thus by producing large distortion (see Japanese Application Patent Laid-Open Publication No. 2005-183478). Though this laminating structure increases emission sensitivity in accordance with the number of lamination layers, however, receiving sensitivity is inversely proportional to the number of lamination layers, which is undesirable to harmonic imaging technology.